1. Field of the Invention
The present invention relates to a photographing device for improving accuracy of a photographic image by correcting image vibration caused by camera vibration due to movement of the hands (hand vibration), and a method for acquiring a photographic image.
2. Description of the Related Art
With current cameras, all important tasks, such as determining an exposure time, focus adjustment, and the like for photographing, are automatically performed, and the probability of taking bad shots is greatly reduced, even for users unfamiliar with camera operations. Moreover, recently, a vibration-correction system for preventing image vibration caused by camera vibration due to movement of the hands (hand vibration) has been studied, and almost all factors causing bad photography have been removed.
Now, description will be made briefly regarding a vibration-correction system for preventing hand vibration from affecting the picture taken.
Hand vibration applied to a camera at the time of photographing commonly has a vibration frequency in the range of about 1 to 10 Hz, and as a basic concept for acquiring a photographic image without image vibration even when such hand vibration is applied to the camera at the time of an exposure, there is an optical vibration-correction system wherein vibration of the camera due to hand vibration is detected, and a compensating lens is displaced within a face orthogonal to the optic axis of the camera in accordance with the detected results.
That is to say, in order to take a picture having no image vibration, even if camera vibration due to movement of the hands occurs, there is a need to, first, precisely detect vibration of the camera due to hand vibration, and then to correct change in the optic axis of the camera due to the hand vibration.
In theory, image vibration can be corrected by including in a camera a vibration detecting unit including a correction lens, for detecting acceleration, angular acceleration, angular speed (velocity), and angular displacement and the like using an accelerometer, an angular accelerometer, a vibration gyro, a laser gyro, or the like, making computations in accordance with the detected results as appropriate, and driving a vibration-correction optical device for de-centering the photographing optic axis of the camera based on the detected information of camera vibration due to movement of the hands from the vibration detecting unit.
Japanese Patent No. 3,110,797 discloses a method for acquiring a photographic image with a long exposure time by repeating photographing multiple times, each of which is so short that camera vibration due to movement of the hands is negligible in each, and synthesizing the multiple images thus obtained while correcting offset in each image, i.e., as a synthesized image.
The latest digital cameras have become more compact than silver-salt compact cameras. In particular, cameras including image sensors of VGA class have become sufficiently compact in size so as to be built into portable electronic apparatuses (for example, a cell phone).
Under such circumstances, in the event an optical vibration-correction system is included in a camera, there is a need to further reduce in size the vibration-correcting optical device or to reduce in size the vibration detecting unit.
However, in a vibration-correcting optical device, reduction in size is restricted because there is a need to support and drive the compensating lens with high precision. Currently employed vibration detecting units mostly utilize inertial force, thereby leading to a problem wherein reducing the size of the vibration detecting unit deteriorates detecting sensitivity, so high-precision vibration correction cannot be performed.
Furthermore, examples of vibration applied to a camera include angular vibration of which the center is a predetermined axis, and shift vibration in which a camera vibrates in the horizontal direction, and though the optical vibration-correction system can correct angular vibration, it cannot handle shift vibration because this system utilizes inertial force. In particular, there is a tendency that the more compact a camera becomes, the larger this shift vibration becomes.
On the other hand, as another vibration-correction system, there is a method for acquiring vibration-free animation by detecting a movement vector of an image based on the output of the imaging element, as employed in motion photographing using a video camera, and changing a read-out position of the image so as to match the movement vector.
In a case using such a method, there is an advantage wherein the entire product can be reduced in size because a dedicated vibration detecting unit such as the above-described vibration-correcting optical device and the compensating lens are not necessary. However, this vibration-correction system for a video camera cannot be simplistically applied to a digital camera. This reason will be described below.
A movement vector in a video camera is extracted each time an image is read out, for example, in the event that 15 frame images are extracted per second, a movement vector is detected by comparing these extracted images. However, in the event of taking a still image using a digital camera, a movement vector cannot be detected by comparing images as with a video camera because the digital camera performs a single exposure as to a subject. Accordingly, simple application of a vibration-correction system for a video camera cannot be made to a digital camera.
On the other hand, with a vibration-correction method as disclosed in Japanese Patent No. 3,110,797, photographing is repeated multiple times, so that photographing is performed over a longer span of time. Accordingly, there is the possibility that needlessly employing this method increases unintended bad photographing, such as moving of the subject. The user needs to pay attention to conditions for employing this vibration-correction method, thereby leading to a hard-to-handle system as compared with a common photographing method for performing photographing using a single exposure. The following will be described in detail.
With a digital camera, unlike a silver-salt camera, sensitivity of image sensors (imaging sensitivity) can be freely changed. Thus, improving imaging sensitivity even in a dark subject enables a certain level of vibration affecting an image to be prevented. Photographing sensitivity can be set automatically by automatic setting of the camera, or set manually by a user himself/herself.
As described above, with a digital camera, there are more parameters to determine the necessity of vibration-correction than those of a silver-salt camera, so that it is harder than ever for a user himself/herself to determine whether or not vibration-correction is necessary, thereby leading to a hard-to-handle system for the user.
Moreover, there is the possibility that the above-described vibration-correction method may bring the user discomfort, since multiple exposures are performed each time photographing is performed.
Furthermore, with this vibration-correction method, though displacement between images is determined by change in characteristic points on the images so as to correct offset of the images, the offset between the images cannot be precisely corrected in the event that characteristic points on the images cannot be extracted in a reliable manner, or in the event that displacement of the characteristic points is not detected even if the characteristic points on the images can be identified.
That is to say, in the event that an incorrect characteristic point is extracted, excessive correction is performed even though hand vibration is not occurring in reality, thereby leading to the possibility that an image having a large level of vibration is unintentionally created.
Also, though images are synthesized without correction when displacement of characteristic points cannot be detected, image vibration due to movement of the hands (hand vibration) occurs in a synthesized image, which is often larger than movement of the subject. This is because an exposure time required for performing divided multiple exposures is longer than an exposure time required for performing a single exposure so as to obtain an appropriate exposure. Accordingly, there is the possibility that performing multiple exposures in order to perform vibration-correction will unintentionally create an image with more blurring.